Linked to the current and pending rises in the price of electricity, and the fact that water consumption will become an increasingly sensitive issue in Southern Africa, mineral processing research is starting to focus on rock sorting prior to crushing and milling.
If a mineral processing operation can reject 20% to 30% of the, say, 100 mm to 150 mm size material that does not contain the mineral/s of interest, prior to the crushing and milling phase, the savings on water and power consumption will be significant.
Sorting of material prior to the comminution circuit is not a new concept in South Africa. “During the 1960s and 70s gold mining operations made use of manual labour to sort waste rock from the ore based on the colour of these rocks,” Dr Roger Paul, Mintek’s general manager of technology says. “The sorters took advantage of the fact that when sulphides are present, with which the gold is associated, the rock has a different colour from pure quartzite.”
Later radiometry was used, since gold is associated with uranium in pyrite in South Africa, and sorting by this method allowed for the identification of the gold and uranium containing material. “However, the detection rates were poor and small rocks that were of value could easily be passed over.”
In the 1980s radiometric sorting was largely abandoned, mainly because the throughput was not fast enough. Rocks were pushed through in a single line and sorted individually with a gate system. At the same time the industry moved onto large processing plants thanks to the introduction of the now established CIL/CIP technology, and the processing of run-of-mine ore became the standard.
However, since those days, the bandwidths monitored by the various sensing systems now cover the whole electromagnetic spectrum, and the quality of the sensors used for detection are greatly superior, having improved dramatically over the past few decades. This makes the detection and sorting of material worth reconsidering.
A number of techniques are used, including radiometric, X-ray fluorescence, X-ray transmission, optical colour sorting, near infra red detection and magnetic/conductive sorting. The other aspect of the technology required is that once the identification is done, the electromechanical sorting system must be able to perform the separation task efficiently.
Mintek is investigating sorting technologies, and has had two different systems on trial. These are respectively known as the Commodas UltraSort system originating in Germany and the Rados system originating in Russia.
The Commodas technology is the more sophisticated of the two and is based on a row of high pressure air jets. Rocks in the system arrive at a 1.0 to 1.5 metre wide channel and fall into space. A row of sensors determines whether the rock should be classified as ore or waste and be blown to another bin accordingly, with the air jets using pressures of about 10 bar.
In comparison, the Russian Rados technology is less technologically complex. It features an incoming feed to separate the rocks, with some four channels featuring flipper paddles and bins.
“Thus we have one system which is sophisticated but higher maintenance and another which is more rugged. We suspect each will be suited to certain applications,” Paul says. “We ordered both units in pilot plant format to assess the strengths and weaknesses of these technologies.”
He sees a number of potential applications, which have to be assessed on a case by case basis. One of these is in the upgrade of manganese ore, where one is looking to reject particles containing high iron content before further processing. “The colour and density is similar, but the X-ray fluorescence of manganese is different from that of iron.”
Paul also sees potential for the sorting of copper ores, which is of particular interest in countries such as Zambia and the DRC. “The brightly coloured copper-containing material is visibly different to that of other rock, and colour sorting or X-ray fluorescence could be used to upgrade the copper ore prior to crushing.”
Another potential application is the sorting of material in the large waste rock dumps that exist at gold mines, and have grades of about 0.6 g/t. These dumps are created during the development stage when waste rock, containing between 0.5 g/t and 0.8 g/t of gold, is put aside.
Paul also sees opportunities in the sorting of coal and shale using X-ray transmission, where here the coal is the material with the lower atomic mass (carbon). “X-ray transmission could be used. Beams passed through the material will detect the shale based on its spectrometric identification, shale having a much higher atomic mass than the surrounding carbon material,” Paul says.
With the increase in interest in uranium recovery, and with uranium present in low concentrations in South African ores, the opportunity to sort a percentage of the feed before comminution could be the difference between profitable recovery and a non-viable project. Here X-ray fluorescence sorting or radiometric sorting could be used.
Paul says that Mintek’s investigations are seeing where and if rock sorting can work with South and Southern African ore types, and what applications will turn out to be worthwhile.
He says that sorting can be done for material ranging from 10 mm to 150 mm in size, though taking into account that the size feed ratio probably should not exceed 4:1. “In other words if the top size is 100 mm, then the bottom cut off will be about 25 mm. Similarly you could have 25 mm as a top size and a bottom size cut-off of about 6.0 mm.” Having material segregated into two streams using two sorters is also a possibility.
“According to our data, there are some 300 Rados and Commodas units in operation around the world. They are quite widely used in Russia, taking into account that operations in geographically isolated regions such as Siberia rely on diesel gensets for power, and the associated cost sensitivities.”
In assessing the suitability of the application of these technologies in this region, the speed of sorting will be important. However, after the large plants introduced in the 1970s onwards saw sorting phased out, clearly the time has come for a reassessment of this option. The renewed focus on energy costs, which in the past were not seen as important, together with technological advances, makes the reintroduction of sorting systems once again a topic that deserves attention.